When detecting accident situations of passenger cars, the solid-borne or structure-borne sound measurement represents anew technology. The airbag control device evaluates the structural and solid-borne sound vibrations arising in a crash and enables a fast, targeted triggering of the restraint devices.
For example, in DE 10015273 A1 already the evaluation of solid-borne sound is detected as high-frequency vibrations by a broadband sensor and from this a low-frequency acceleration component and a high-frequency solid-borne sound signal component is generated. Frequencies above 4 kHz shall be allowed to pass from a high-pass filter into the evaluation path for the solid-borne sound signal. The passband of a high-pass filter, however, requires a −3 dB cut-off frequency at or below 4 kHz for simple filters, as otherwise the signal would already be significantly damped. Or significantly more complex high-order high-pass filters would be required.
The previous filtering of solid-borne sound signals is usually effected via a bandpass, rectifying and low-pass and has some further disadvantages. These include the relatively long filter time in the sensor, which can be a problem precisely in side impact tests which are to be triggered quickly. Due to the low-pass in the current signal processing a signal is additionally delayed. The entire filter chain implicates that a high-frequency signal arrives delayed and only very strongly smoothed in the evaluation logic of an occupant protection system.